1. Field of the Invention
The present invention relates to a method and apparatus for warning a vehicle operator of the need to replace a transmission oil filter due to filter restriction or filter bypassing.
2. Description of the Related Art
The hydrostatic unit of an infinitely variable, hydro-mechanical transmission is susceptible to damage if the oil is contaminated by particles. Oil filters are used to remove contaminants from the oil. However, to enable continued operation of the vehicle, such as an agricultural tractor, a bypass valve is also provided that will automatically open in the event the filter is excessively restricted. The current state of the art for detecting filter restriction is with a pressure switch that only indicates when the filter is restricted by more or less than a fixed value. The switch can not indicate whether or not the filter is restricted to the point of bypass. As a result, the transmission controller can not do any more to protect the transmission than to turn on an indicator light to signal a restricted filter. If the operator chooses to ignore the warning, then the vehicle can continue to be operated until the filter is bypassed and debris is passed through sensitive hydraulic components.
It is an object of the present invention to provide a method and apparatus for warning the operator that the filter has become restricted, for further warning if the filter is bypassed and for taking preventative measures to close the bypass valve before any damage occurs.
The transmission is equipped with a pair of pressure transducers in addition to the filter and the bypass valve. One pressure transducer measures the pump pressure at the pump outlet while the other transducer is at the filter outlet. In a preferred embodiment, the second pressure transducer is located adjacent the park brake to measure the pressure in the park brake piston. However, this second pressure transducer can be located anywhere that will measure the pressure at the filter outlet. The outputs of the two pressure transducers are directed to an electronic controller that compares the two pressure measurements and takes appropriate action to activate alarms, depending on the magnitude of the pressure differential across the filter.
A first alarm will be activated when the filter is partially restricted, enabling the operator to continue operating the tractor and schedule maintenance. If the pressure differential exceeds a second higher level, indicating a bypassed filter, the controller will activate a second, more serious, alarm. The controller will also reduce the engine speed, thereby reducing the pump outlet pressure whereby the pressure differential across the filter will hopefully be reduced to the point that the bypass valve will close. In the event the bypass valve does not close, the controller will reduce the engine speed to a low idle until the engine is shut down and will shift the transmission to neutral, thereby forcing action to be taken. This prevents continued operation of the transmission with the bypass valve opened and the possibility for damage of the hydrostatic unit.
The alarms are only activated when the oil temperature is above a predetermined level, such as 20xc2x0 C. A greater pressure differential across the filter will occur due to the higher viscosity of cold oil. Thus, when the oil is cold the pressure differential is not indicative of a restricted or bypassed filter.
The use of two pressure transducers produces an accurate measure of the extent of filter restriction. An accurate measurement allows for the use of more sophisticated software to activate alarms rather than a simple differential pressure switch. Furthermore, a downstream pressure transducer can be used to diagnose leaks or other problems in the transmission. Multiple alarms may be activated for varying degrees of filter restriction instead of the single, restricted filter alarm previously known. A gauge display can also be employed to show the ongoing filter status.